1. Field of the Invention
The present invention relates generally to the technical field of mobile communications, and specifically to base station apparatuses and methods used in mobile communications system using next-generation mobile communications techniques.
2. Description of the Related Art
In this type of technical field, mobile communications schemes to succeed so-called third generation mobile communications schemes is being studied by 3GPP, a standardization body for Wideband code division multiple access (W-CDMA) schemes. More specifically, Long term evolution (LTE) schemes are being studied at a feverish pace to succeed the W-CDMA, high-speed downlink packet access (HSDPA), and high-speed uplink packet access (HSUPA) schemes. A downlink radio access scheme in the LTE is Orthogonal frequency division multiple access (OFDMA). For uplink, Single-carrier frequency division multiple access (SC-FDMA) is used. (See Non-patent document 1, for example.)
The Orthogonal frequency division multiple access (OFDMA) is a scheme which divides a frequency band into multiple narrow frequency bands (sub-carriers) and overlaying data onto the respective sub-carriers to transmit the overlaid data. Densely lining up the sub-carriers such that they are in an orthogonal relationship with each other on the frequency axis makes it possible to expect that high-speed transmission be achieved and frequency utilization efficiency be increased.
The Single-carrier frequency division multiple access (SC-FDMA) is a single-carrier transmission scheme which divides a frequency bandwidth per terminal and uses mutually different frequency bands among multiple terminals to conduct transmission. Such a scheme as described above, which makes it possible to easily and effectively reduce inter-terminal interference as well as to reduce fluctuations in transmission power, is preferable from points of view of reducing power consumption of terminals and widening the coverage, etc.
In the LTE system, for downlink or uplink, one or more resource blocks (RBs) or resource units (RUs) are allocated to a user apparatus to conduct communication. For convenience of explanations, resource blocks and resource units, which are used interchangeably, both represent units of frequencies for resource allocation. The resource blocks are shared by a large number of user apparatuses within a system. As an example, one resource block has a bandwidth of 180 kHz. For example, 25 resource blocks are included in a system bandwidth of 5 MHz. For example, in the LTE, a base station apparatus determines which user apparatus of multiple user apparatuses a resource block is allocated to for each sub-frame, which is 1 ms. The sub-frame may be called a transmission time interval (TTI). Determining allocations of radio resources is called scheduling. In downlink, abase station apparatus transmits, to a user apparatus selected in the scheduling, a shared channel in one or more resource blocks. The shared channel is called a physical downlink shared channel (PDSCH). In uplink, the user apparatus selected in the scheduling transmits, to the base station apparatus, a shared channel in one or more resource blocks. The shared channel is called a physical uplink shared channel (PUSCH).
In a communications system using the shared channels as described above, it is necessary to signal, for each sub-frame, which user apparatus the shared channel is allocated to. A control channel used in the above-mentioned signaling is called a physical downlink control channel (PDCCH) or a downlink (DL)-L1/L2 control channel. In addition to the PDCCH, a downlink control signal may include a physical control format indicator channel (PCFICH) and a physical hybrid ARQ indicator channel (PHICH).
The PDCCH may include the following information sets, for example. (See Non-patent document 2, for example.):
downlink scheduling information;
an uplink scheduling grant;
a transmission power control command bit.
The downlink scheduling information includes information on downlink shared channel, for example, and, more specifically, includes information on allocating downlink resource block, information on identifying user apparatus (UE-ID), the number of streams, information on pre-coding vector, data size, modulation scheme, information on HARQ (hybrid automatic repeat request), etc.
Moreover, the uplink scheduling grant includes information on uplink shared channel, for example, and, more specifically, includes information on allocating uplink resource, information on identifying user apparatus (UE-ID), data size, modulation scheme, information on uplink transmission power, information on demodulation reference signal in uplink MIMO (multiple-input multiple-output), etc.
The PCFICH is information for reporting the PDCCH format. More specifically, the number of OFDM symbols mapped to the PDCCH is reported using the PCFICH. In the LTE, the number of OFDM symbols mapped to the PDCCH is 1, 2 or 3, which mapping being performed in order from a beginning OFDM symbol of a sub-frame. The PCFICH may be defined as an information set which constitutes a part of the PDCCH or an information set which is different from the PDCCH. The PCFICH is described in Non-patent document 3, for example.
The PHICH includes acknowledgement/non-acknowledgement information (ACK/NACK), which indicates whether retransmission is needed for the PUSCH transmitted in uplink.
For definition of terms, the PDCCH, PCFICH and PHICH may be defined as respectively independent channels, or the PDCCH may be defined to include the PCFICH and PHICH.
In uplink, user data (a normal data signal) and accompanying control information is transmitted using the PUSCH. Moreover, separately from the PUSCH, downlink quality information (CQI; channel quality indicator) and PDSCH acknowledgement/non-acknowledgement information (ACK/NACK), etc. are transmitted using the physical uplink control channel (PUSCH). The CQI is used for downlink physical shared channel scheduling process, adaptive modulation/demodulation and channel encoding process (AMC), etc. In uplink, a random access channel (RACH) and a signal indicating a request for allocating uplink and downlink radio resources are also transmitted as needed.
Now, in resource block scheduling, for each sub-frame, for each user, and, for each resource block, a certain metric Mu,f (i) is calculated, and the calculated results are compared with one another. Here, i represents a sub-frame, u represents a user, and F represents a resource block (frequency). Then, a resource block is allocated preferentially to a user indicating a greater metric value. Moreover, transmission formats (data modulation scheme and channel encoding rate (or data modulation scheme and data size)) are determined based on channel conditions. Normally, in scheduling, an amount representing channel conditions (SINR (signal-to-interference plus noise power ratio)) is included in the metrics. Various metrics are used depending on how the scheduling is performed. For example, from a viewpoint of maximizing a system throughput, a maximum CI method is used, where the metric Mu,f(i) is given by the equation:Mu,f(i)=γu,f(i)The right side represents an instantaneous received SINR in a sub-frame i for a user u. For convenience of explanations, uplink scheduling is assumed. It is always a user with good channel conditions that conducts communications, the throughput becomes maximal. However, this lacks fairness among the users. Then, there is also a scheduling method called a proportional fairness (PF) method. In the PF method, the following metric is used.Mu,f(i)=γu,f(i)/E(γu,f)Here, E means averaging. The averaging in this case means that the effect of shadowing or of distance fluctuation remains, but the effect of instantaneous fading is smoothed.
The PF method is the same as the maximum CI method in allocating resources to a user with a greater metric Mu,f(i), but is different from the maximum CI method in that an average value of received qualities for the user is also taken into consideration. With this method, an opportunity for resource allocation is provided once the channel condition of each user becomes better than an average channel condition for each user apparatus. Thus, an improvement in the throughput may be achieved to some extent while correcting for unfairness due to the maximum CI method.
Non-patent document 1: 3GPP TR 25.814 (V7.1.0), “Physical Layer Aspects for Evolved UTRA,” October 2006
Non-patent document 2: R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding
Non-patent document 3: 3GPP TR36.211 (V8.1.0), “Physical Channels and Modulation”, December 2007